Question is, can I still add the 5th M.2 SSD to x4 PCI-E with adaptor while other 4 M.2 SSD is already in there?

I don't know which kind of adapter you mean, but if you've got one, then yes. Why not? Maybe just try it?
Anyhow, you know that populating DIMM.2 slot puts the 16x PCI-E to 8x anyway? In the way the board was designed, there's no other way because these consumer CPUs only provide 16 direct PCI-E lanes. Ok?

I don't know which kind of adapter you mean, but if you've got one, then yes. Why not? Maybe just try it?
Anyhow, you know that populating DIMM.2 slot puts the 16x PCI-E to 8x anyway? In the way the board was designed, there's no other way because these consumer CPUs only provide 16 direct PCI-E lanes. Ok?

SilenMar,
Yes you can use either an adapter card or a pcie drive, I have an Intel 900 in pcie x4 slot, 2 Samsung 950's on board slots, a Samsung 960 and Plextor drives on the DIMM.2 module on my XI Gene